1. Field of the Invention
The present invention relates to a dispersion-compensating optical fiber which compensates for the chromatic dispersion and dispersion slope of a dispersion-shifted optical fiber, an optical transmission line including the dispersion-shifted optical fiber and dispersion-compensating optical fiber, and a dispersion-compensating module formed by winding the dispersion-shifted optical fiber like a coil.
2. Related Background Art
For achieving further higher speed and larger capacity in optical transmission systems which carry out wavelength division multiplexing (WDM) optical transmission, it is important that the absolute value of accumulated chromatic dispersion be as small as possible in a wide signal wavelength band. In general, since it is hard to achieve in an optical transmission line using only one kind of optical fiber, a plurality of kinds of optical fibers have been connected so as to construct an optical transmission line, thereby lowering the absolute value of accumulated chromatic dispersion in the optical transmission line in a wide band.
For example, Japanese Patent Application Laid-Open No. HEI 6-11620 discloses a technique in which a standard single-mode optical fiber (SMF) having a zero-dispersion wavelength near a wavelength of 1.3 xcexcm and a dispersion-compensating optical fiber (DCF) which compensates for the chromatic dispersion of this standard single-mode optical fiber at a wavelength of 1550 nm are connected to each other, so as to reduce the absolute value of accumulated chromatic dispersion of the optical transmission line constructed by thus connected optical fibers in a 1.55-xcexcm wavelength band.
Also, U.S. Pat. No. 5,838,867 discloses a technique in which a non-zero dispersion-shifted optical fiber (NZDSF) having a small positive chromatic dispersion at a wavelength of 1550 nm and a dispersion-compensating optical fiber which compensates for the chromatic dispersion and dispersion slope of this dispersion-shifted optical fiber are connected to each other, so as to lower the absolute value of accumulated chromatic dispersion of the optical transmission line constructed by thus connected optical fibers in a 1.55-xcexcm wavelength band.
Here, the chromatic dispersion of a standard single-mode optical fiber (SMF) at a wavelength of 1550 nm is referred to as DSMF, and its dispersion slope is referred to as SSMF. The chromatic dispersion of a non-zero dispersion-shifted optical fiber (NZDSF) at a wavelength of 1550 nm is referred to as DDSF, and its dispersion slope is referred to as SDSF. The chromatic dispersion of a dispersion-compensating optical fiber (DCF) at a wavelength of 1550 nm is referred to as DDCF, and its dispersion slope is referred to as SDCF. Then, for reducing the absolute value of accumulated chromatic dispersion of an optical transmission line in a wide band including a wavelength of 1550 nm, it is required in a dispersion-compensating optical fiber (hereinafter referred to as xe2x80x9cdispersion-compensating optical fiber for SMFxe2x80x9d) for compensating for both of the chromatic dispersion and dispersion slope of the single-mode optical fiber that its ratio (SDCF/DDCF) of dispersion slope SDCFto the chromatic dispersion DDCF be substantially equal to the ratio (SSMF/DSMF) of dispersion slope SSMF to the chromatic dispersion DSMF of the single-mode optical fiber. Also, it is required in a dispersion-compensating optical fiber (hereinafter referred to as xe2x80x9cdispersion-compensating optical fiber for DSFxe2x80x9d) for compensating for both of the chromatic dispersion and dispersion slope of the dispersion-shifted optical fiber that its ratio (SDCF/DDCF) of dispersion slope SDCF to the chromatic dispersion DDCF be substantially equal to the ratio (SDSF/DDSF) of dispersion slope SDSF to the chromatic dispersion DDSF of the dispersion-shifted optical fiber.
The inventors have studied conventional optical transmission lines in detail and, and as a result, have found problems as follows.
As compared with standard single-mode optical fibers, dispersion-shifted optical fibers have a greater ratio (SDSF/DDSF) at a wavelength of 1550 nm. Therefore, it is necessary for dispersion-compensating optical fibers for DSF to have a greater ratio (SDSF/DDSF) at the wavelength of 1550 nm as compared with dispersion-compensating optical fibers for SMF.
The dispersion-compensating optical fiber for SMF a disclosed in Japanese Patent Application Laid-Open. No. HEI 6-11620 compensates for the chromatic dispersion of a standard single-mode optical fiber having a zero-dispersion wavelength near a wavelength of 1.3 xcexcm and a large chromatic dispersion at a wavelength of 1550 nm, and has a negative chromatic dispersion with a large absolute value. Therefore, this dispersion-compensating optical fiber for SMF is suitable for compensating for the chromatic dispersion of the standard single-mode optical fiber. However, this dispersion-compensating optical fiber for SMF is not sufficient for compensating for the dispersion slope.
On the other hand, the dispersion-compensating optical fiber for DSF disclosed in U.S. Pat. No. 5,838,867 can compensate for both of the chromatic dispersion and dispersion slope of a non-zero dispersion-shifted optical fiber having a small positive chromatic dispersion at a wavelength of 1550 nm. Since this dispersion-compensating optical fiber for DSF has a chromatic dispersion with a small absolute value, a long dispersion-compensating optical fiber for DSF is needed for compensating for both of the chromatic dispersion and dispersion slope of the non-zero dispersion-shifted optical fiber.
For example, the non-zero dispersion-shifted optical fiber disclosed in the document 1, S. Bigo, et al., xe2x80x9c1.5 Terabit/s WDM transmission of 150 channels at 10 Gbit/s over 4xc3x97100 km of TeraLight(trademark) fibre,xe2x80x9d ECOC""99, PD (1999), has a chromatic dispersion of +8 ps/nm/km and a dispersion slope of +0.06 ps/nm2/km at a wavelength of 1550 nm. On the other hand, the non-zero dispersion-shifted optical fiber disclosed in the document 2, D. W. Peckham, et al., xe2x80x9cReduced dispersion slope, non-zero dispersion fiber, xe2x80x9d ECOC""98, pp. 139-140 (1998), has a chromatic dispersion of +4 ps/nm/km and a dispersion slope of +0.046 ps/nm2/km at a wavelength of 1550 nm. For compensating for both of the chromatic dispersion and dispersion slope of any of the non-zero dispersion-shifted optical fibers disclosed in these documents having a length of 80 km, a dispersion-compensating optical fiber for DSF having a length of 8 km to 16 km is necessary.
Meanwhile, even upon slight bending, fundamental-mode light is likely to leak out from dispersion-compensating optical fibers for DSF in general, so that bending loss is large in the fundamental-mode light. Consequently, transmission loss becomes greater when a dispersion-compensating optical fiber for DSF is laid as being cabled or is formed into a dispersion-compensating module as being wound like a coil. Therefore, in an optical transmission system which carries out optical communications by propagation of signals through an optical transmission line constructed by a dispersion-shifted optical fiber and a dispersion-compensating optical fiber for DSF which are connected to each other, transmission loss is so large in the optical transmission line that its repeater section cannot be elongated, whereby higher speed and larger capacity cannot further be achieved in optical communications.
In order to overcome the problems mentioned above, it is an object of the present invention to provide a dispersion-compensating optical fiber which can compensate for the chromatic dispersion and dispersion slope of a non-zero dispersion-shifted optical fiber by a short length, an optical transmission line with a low transmission loss including the dispersion-shifted optical fiber and dispersion-compensating optical fiber, and a dispersion-compensating module with a low transmission loss in which the dispersion-compensating optical fiber is wound like a coil.
The dispersion-compensating optical fiber according to the present invention has, at a wavelength of 1550 nm, a chromatic dispersion DDCF of xe2x88x9240 ps/nm/km or less and a ratio (SDCF/DDCF) of dispersion slope SDCF to the chromatic dispersion DDCF of 0.005/nm or more. By this structure, the occurrence of non-linear phenomenon can be effectively prevented because an effective area can be easily expand. Preferably, in the dispersion-compensating optical fiber according to the present invention, the chromatic dispersion DDCF is xe2x88x92100 ps/nm/km or more but xe2x88x9240 ps/nm/km or less, and the ratio (SDCF/DDCF) of dispersion slope SDCF to the chromatic dispersion DDCF is 0.005/nm or more but 0.015/nm or less. Since the chromatic dispersion DDCF is negative with a large absolute value, and the ratio (SDCF/DDCF) of dispersion slope SDCF to chromatic dispersion DDCF lies within the numerical range mentioned above, this dispersion-compensating optical fiber can compensate for the chromatic dispersion and dispersion slope of a dispersion-shifted optical fiber in a wide band including a wavelength of 1550 nm by a short length.
The dispersion-compensating optical fiber according to the present invention preferably has an effective area of 16 xcexcm2, more preferably 20 xcexcm2 or more at the wavelength of 1550 nm. In this case, it can restrain four-wave mixing from occurring and suppress deterioration in the waveforms of light signals propagating therethrough.
The dispersion-compensating optical fiber according to the present invention preferably has a cutoff wavelength of 1.2 xcexcm or more but 1.8 xcexcm or less, more preferably 1.4 xcexcm or more but 1.8 xcexcm or less. Also, the dispersion-compensating optical fiber according to the present invention preferably has a transmission loss of 0.5 dB/km or less at a wavelength of 1550 nm. In this case, bending loss can be restrained from increasing since the cutoff wavelength is longer than that, conventionally obtained, and lower loss is attained even when the optical fiber is formed into a cable or module, since the transmission loss lies within the numerical range mentioned above as well.
The dispersion-shifted optical fiber according to the present invention preferably has a core region extending along a predetermined axis and having a first refractive index, and a cladding region surrounding around the outer periphery of the core region. The cladding region includes a first cladding surrounding around the outer periphery of the core region and having a second refractive index lower than the first refractive index, a second cladding surrounding around the outer periphery of the first cladding and having a third refractive index higher than the second refractive index, and a third cladding surrounding around the outer periphery of the second cladding and having a fourth refractive index lower than the third refractive index. The core region preferably has a relative refractive index difference of 0.8% or more but 2.0% or less, more preferably 0.8% or more but 1.5% or less, with reference to the fourth refractive index of the third cladding. The first cladding preferably has a relative refractive index difference of xe2x88x920.4% or lower with reference to the fourth refractive index of the third cladding. These cases are suitable for realizing a dispersion-compensating optical fiber having the above-mentioned characteristics.
In the dispersion-compensating optical fiber according to the present invention, the ratio (SDCF/DDCF) preferably changes by 10% or less when the outside diameter of the second cladding changes by 2%. In this case, a dispersion-compensating optical fiber having a desirable chromatic dispersion characteristic can be made easily.
On the other hand, for reduction of a transmission loss and fiber non-linearity by shortening a fiber length, the dispersion-compensating optical fiber according to the present invention preferably has the chromatic dispersion DDCF is xe2x88x92250 ps/nm/km or more but xe2x88x92120 ps/nm/km or less, the ratio (SDCF/DDCF) of dispersion slope SDCFto the chromatic dispersion DDCF is 0.005/nm or more, and an effective area of 10 xcexcm2 or more but 20 xcexcm2 or less, more preferably (20xe2x88x92|DDCF|/25) or more but (23xe2x88x92|DDCF|/25) or less. Because it is necessary to reduce a size of the effective area at a bending weak portion where an absolute value of dispersion becomes large. Also, the dispersion-compensating optical fiber preferably has a transmission of 1.0 dB/km or less. As described above, the dispersion-compensating optical fiber has the core region, and the cladding region including the first to third cladding. And, it is preferably that a relative refractive index difference of the core region with respect to the third cladding is 2.0% or more but 3.0% or less, and that a relative refractive index difference of the first cladding with respect to the third cladding is xe2x88x920.4% or less.
The optical transmission line according to the present invention has a repeater section laid with the above-mentioned dispersion-compensating optical fiber; and a dispersion-shifted optical fiber fusion-spliced to the dispersion-compensating optical fiber. The dispersion-shifted optical fiber has, at a wavelength of 1550 nm, a chromatic dispersion of +2 ps/nm/km or more but +10 ps/nm/km or less, and a dispersion slope of +0.04 ps/nm2/km or more but +0.12 ps/nm2/km or less. When the dispersion-shifted optical fiber and the dispersion-compensating optical fiber are connected to each other with an appropriate length ratio, thus formed optical transmission line yields, on the whole, an average chromatic dispersion with a small absolute value and an average dispersion slope with a small absolute value at the wavelength of 1550 nm. As a consequence, this optical transmission line has, on the whole, an average chromatic dispersion with a small absolute value and a small average transmission loss in a wide wavelength band including a wavelength of 1550 nm.
The optical transmission line according to the present invention preferably has, on the whole, an average chromatic dispersion with a deviation (=maximum valuexe2x88x92minimum value) of 0.2 ps/nm/km or less in a wavelength band of 1535 nm or more but 1560 nm or less (C band). More preferably, the average chromatic dispersion on the whole has a deviation of 0.2 ps/nm/km or less in a wavelength band of 1535 nm or more but 1600 nm or less (C and L bands). In an optical transmission system which carries out optical communications by making signals propagate through such an optical transmission line in this case, the optical transmission line yields a low average transmission loss, the average chromatic dispersion has a small absolute value, and optical transmission with a high bit rate is possible in a wide wavelength band (including at least the C band and further the L band) including the wavelength of 1550 nm. Therefore, this optical transmission system can elongate the repeater section and achieve further higher speed and larger capacity in optical communications.
The dispersion-compensating module according to the present invention is characterized in that the above-mentioned dispersion-compensating optical fiber is wound like a coil so as to form a module. The dispersion-compensating module in which the dispersion-compensating optical fiber is formed into a module compensates for the chromatic dispersion and dispersion slope of a dispersion-shifted optical fiber laid in a repeater section, and yields, on the whole, an average chromatic dispersion with a small absolute value and an average dispersion slope with a small absolute value at a wavelength of 1550 nm when the dispersion-shifted optical fiber and dispersion-compensating optical fiber have an appropriate length ratio therebetween. As a consequence, the whole of the dispersion-shifted optical fiber and dispersion-compensating module have an average chromatic dispersion with a smaller absolute value and a small average transmission loss in a wide wavelength band including the wavelength of 1550 nm.
The dispersion-compensating module according to the present invention preferably has a total loss of 7 dB or less in a wavelength band of 1535 nm or more but 1565 nm or less, more preferably a total loss of 7 dB or less in a wavelength band of 1535 nm or more but 1610 nm or less, when yielding a dispersion-compensating amount of xe2x88x92640 ps/nm at the wavelength of 1550 nm. In the dispersion-compensating module according to the present invention, the total loss is preferably 3 dB or less in the wavelength band of 1535 nm or more but 1565 nm or less, more preferably 3 dB or less in the wavelength band of 1535 nm or more but 1610 nm or less, when the dispersion-compensating amount is xe2x88x92320 ps/nm at the wavelength of 1550 nm. In an optical transmission system having this dispersion-compensating module, its average transmission loss is small, its average chromatic dispersion has a small absolute value, and optical transmission with a high bit rate is possible in a wide wavelength band (wavelength band including at least the C band and further the L band) including the wavelength of 1550 nm. As a consequence, this optical transmission system can elongate the repeater section and achieve further higher speed and larger capacity in optical communications.
Further, the dispersion-compensating optical fiber according to the present invention preferably has, at a wavelength of 1550 nm, a chromatic dispersion DDCF of xe2x88x9240 ps/nm/km or less, a ratio (SDCF/DDCF) of dispersion slope SDCF to the chromatic dispersion DDCF of 0.005/nm or more, and an effective area of 16 xcexcm2 or more, more preferably 20 xcexcm2 or more. In a wide wavelength band including a wavelength of 1550 nm, this dispersion-compensating optical fiber not only can compensate for the chromatic dispersion and dispersion slope of a dispersion-shifted optical fiber by a short length, but also can restrain four-wave mixing from occurring and suppress deterioration in the waveforms of light signals propagating therethrough.
Still further, the dispersion-compensating optical fiber according to the present invention preferably has, at a wavelength of 1550 nm, a chromatic dispersion DDCF of xe2x88x9240 ps/nm/km or less, a ratio (SDCF/DDCF) of dispersion slope SDCF to the chromatic dispersion DDCF of 0.005/nm or more, and a transmission loss of 0.5 dB/km or less. This dispersion-compensating optical fiber not only can compensate for the chromatic dispersion and dispersion slope of a dispersion-shifted optical fiber by a short length, but also yields a low loss even when formed into a cable or module.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.